Acceleration sensor device

ABSTRACT

Providing a small-sized acceleration sensor including a vibrator formed of a piezoelectric single crystal, and a weight section connected to the vibrator and supported at a position different from the position of the center of gravity of an assembly of the vibrator and weight section. Two divided electrodes used for detecting an electrical signal are formed on the vibrator and connected to two wiring patterns of the weight section which also functions as a signal detecting substrate, with an anisotropic conductive adhesive. When an acceleration in one direction is applied, an angular moment exerted in the weight section is detected as sliding vibration by the vibrator, and an electrical signal corresponding to the acceleration is output from the electrodes through the wiring patterns.

This application is a divisional of prior application Ser. No.09/722,716 filed Nov. 28, 2000 now U.S. Pat. No. 6,578,421.

BACKGROUND OF THE INVENTION

The present invention relates to an acceleration sensor for detecting anacceleration, and more particularly to an acceleration sensor fordetecting an acceleration by sliding vibration, which is exerted when anacceleration is applied, and to an acceleration sensor deviceconstructed by storing this acceleration sensor in a package.

An acceleration sensor is installed in equipment to monitor an abnormalcondition of the equipment by detecting an acceleration and vibration ofthe equipment. For example, the acceleration sensor is used to preventerrors in reading and writing data resulting from vibration and shock ina hard disk drive, to prevent hand shaking in a video camera, to actuatean air bag in a vehicle, etc.

With a reduction in size and an increase in the performance of equipmentin which an acceleration sensor is to be installed, there has been ademand for the development of a small-sized, high-performanceacceleration sensor capable of being mounted on a surface of theequipment. As such a small-sized acceleration sensor, an accelerationsensor using a piezoelectric element has been conventionally put intopractice. Disclosed examples of such an acceleration sensor include anacceleration sensor which detects an acceleration by using a deflectionof a piezoelectric single crystal (Japanese Patent Application Laid-OpenNos. 10-206456/1998 and 11-211748/1999) and an acceleration sensor whichdetects an acceleration by using a deflection of piezoelectric ceramic(Japanese Patent Application Laid-Open No. 6-273439/1994).

Acceleration sensors using the deflection of a piezoelectric singlecrystal or the deflection of piezoelectric ceramic as mentioned abovecan improve the detection sensitivity by increasing the deflection toincrease the stress. Thus, in order to improve the detection sensitivityfor high performance, the mass needs to be increased to produce a largerdeflection, causing a problem that the acceleration sensor becomesheavier and larger in size. On the other hand, when the piezoelectricelement is made thicker, it does not easily deflect and causes a problemthat the detection sensitivity is lowered. Then, in order to improve thedetection sensitivity, there have been proposals to make thepiezoelectric element thinner, to stick two pieces of extremely thinpiezoelectric elements together, etc. However, such proposals areassociated with problems that the fabrication process is complicated andthe cost is increased.

Therefore, the applicant of the present invention proposed anacceleration sensor capable of detecting an acceleration with goodsensitivity by a small-sized structure (Japanese Patent ApplicationLaid-Open No. 2000-97707). This acceleration sensor comprises a vibratorand a weight section which is connected and supported at a positiondifferent from the position of the center of gravity of an assembly ofthe vibrator and weight section, and obtains the magnitude of an appliedacceleration by detecting the amount of characteristic (slidingvibration) of the vibrator corresponding to an angular moment exerted inthe weight section when the acceleration is applied. The presentinventors are pursuing the development and improvement of such anacceleration sensor that has a small-sized structure and a highdetection sensitivity without increasing the size of the vibrator toachieve high performance because it detects sliding vibration instead ofdeflective vibration.

BRIEF SUMMARY OF THE INVENTION

The present invention has been achieved in view of the above-mentionedcircumstance, and its object is to provide an acceleration sensor whichis an improvement of the acceleration sensor proposed in Japanese PatentApplication Laid-Open No. 2000-97707 and capable of realizing a furtherreduction in size and cost while maintaining high performance.

Another object of the present invention is to provide an accelerationsensor which does not limit sliding vibration (or, shearing strain) of avibrator for the purpose of preventing a lowering of the detectionperformance.

Still another object of the present invention is to provide anacceleration sensor capable of improving the connectivity between avibrator and a weight section.

Yet another object of the present invention is to provide anacceleration sensor device having a package structure capable of storingsuch an acceleration sensor efficiently without deteriorating itsdetection performance.

An acceleration sensor of the first aspect of the present inventioncomprises a vibrator provided with an electrode and subject to a slidingvibration; and a weight section connected to the vibrator and supportedat a position different from the position of the center of gravity of anassembly of the vibrator and weight section, wherein the weight sectionis provided with a wiring pattern connected to the electrode, thevibrator detects an angular moment about the support point as slidingvibration, which is exerted in the weight section when an accelerationis applied, and an electrical signal corresponding to the appliedacceleration is output from the electrode through the wiring pattern. Inthis structure, the weight section is provided with the wiring patternfor drawing an electrical signal corresponding to an acceleration to bedetected, and also functions as a signal detecting substrate. Therefore,it is not necessary to newly provide a signal detecting substrate,resulting in a simplified structure and a reduction in the cost.

In an acceleration sensor of the second aspect of the present invention,a detection portion for sliding vibration of the vibrator is divided byelectrodes which are a plurality of divided electrode parts. Since thedetection portion for sliding vibration of the vibrator is divided intoa plurality of parts by the electrodes, the detection section of thevibrator can be divided simply, thereby achieving a reduction in thecost.

In an acceleration sensor of the third aspect of the present invention,the detection portion for sliding vibration of the vibrator is dividedby the electrodes which are a plurality of divided electrode parts andby a groove connected to the electrodes. Since the detection portion forsliding vibration of the vibrator is divided into a plurality of partsby the formation of the groove, the detection portion of the vibrator iscertainly divided and a number of acceleration sensors are readilymanufactured, thereby achieving a reduction in the cost.

In an acceleration sensor of the fourth aspect of the present invention,the vibrator has substantially a rectangular parallelepiped shape, andthe groove has a depth of not less than 10% of the thickness of thevibrator. Since the groove is formed to have a depth of not less than10% of the thickness of the vibrator in the process of dividing thedetection portion of the vibrator into a plurality of parts by theformation of the groove, it is possible to achieve not only a reductionin the cost, but also an improvement in the detection sensitivity.

In an acceleration sensor of the fifth aspect of the present invention,a position where the detection portion is divided is a position where acharge distribution by the sliding vibration is substantially zero.Since the detection portion of the vibrator is divided at a positionwhere the angular moment of the weight section is substantially zero,i.e., a position where the charge distribution by the sliding vibrationof the vibrator is substantially zero, it is possible to achieve notonly a reduction in the cost, but also an improvement in the detectionsensitivity.

In an acceleration sensor of the sixth aspect of the present invention,the vibrator and weight section have substantially a rectangularparallelepiped shape, and a length (width) of the vibrator in anacceleration detecting direction is not more than a length (width) ofthe weight section in the acceleration detecting direction. It istherefore possible to reduce an error signal (crosstalk) generated whenan acceleration in a direction different from that of an acceleration tobe detected is applied, and to improve the S/N ratio.

In an acceleration sensor of the seventh aspect of the presentinvention, the weight section has substantially a rectangularparallelepiped shape, and has greater thickness and/or length (width) inan acceleration detecting direction in its portion on one side oppositeto the other side connected to the vibrator than in its portion on theother side. In the weight section, since the thickness and/or the widthof the portion on the side opposite to the vibrator side including thesupport position is made greater, it is possible to increase the angularmoment of the weight section and improve the detection sensitivity.

In an acceleration sensor of the eighth aspect of the present invention,the weight section is constituted by a plurality of members of differentmaterials. It is therefore possible to select the constituent materialsof the weight section so as to increase the angular moment of the weightsection and improve the detection sensitivity.

In an acceleration sensor of the ninth aspect of the present invention,the weight section has a higher density in its portion on one sideopposite to the other side connected to the vibrator than in its portionon the other side. Since the weight section is constructed by using ahigh-dense material for the portion on the side opposite to the vibratorside, the angular moment of the weight section is increased, therebyimproving the detection sensitivity.

In an acceleration sensor of the tenth aspect of the present invention,the vibrator is bonded to the weight section with an anisotropicconductive adhesive. Since the vibrator and the weight section arebonded together with the anisotropic conductive adhesive, it is possibleto prevent a short circuit between the divided electrodes, provide asatisfactory connection between the vibrator and weight section, andachieve conductivity only in a desired direction.

In an acceleration sensor of the eleventh aspect of the presentinvention, the electrical signal is drawn from one of the dividedelectrodes of the vibrator, which is closer to the position of thecenter of gravity. Since an acceleration is not detected by adifferential output of both the divided electrodes of the vibrator (adifferential electrical signal of both the electrodes), but is detectedby only the output (electrical signal) of one of the divided electrodes,which is closer to the position of the center of gravity, the number ofpatterns to be formed in the detection circuit is reduced, therebyachieving a reduction in the cost.

In an acceleration sensor of the twelfth aspect of the presentinvention, the thinnest portion of the weight section having asubstantially rectangular parallelepiped shape is not located on oneside of the position of the vibrator, including the position of thecenter of gravity. Since the thinnest portion of the weight section isplaced on the vibrator or on a position extended from the vibrator in adirection opposite to a direction toward the position of the center ofgravity, it is possible to reduce crosstalk and improve the S/N ratio.

If some object is placed in the groove of the vibrator, when anacceleration is applied, reciprocal sliding vibration of the vibrator islimited and the detection performance deteriorates. Therefore, theinside of the groove should be kept in a hollow state. Hence, in anacceleration sensor of the thirteenth aspect of the present invention,the inside of the groove is kept in a hollow state by bonding thevibrator and weight section with an anisotropic conductive film.Besides, in an acceleration sensor of the fourteenth aspect of thepresent invention, a protruding portion with a width of not less thanthe width of the groove is formed on the weight section, at a positionfacing the groove, and the groove is covered with the protrudingportion, thereby keeping the inside of the groove in a hollow state.According to the thirteenth and fourteenth aspects, there is nopossibility that an adhesive for connecting the vibrator and weightsection runs into the groove, thereby easily maintaining the inside ofthe groove in a hollow state. As a result, reciprocal sliding vibrationof the vibrator is not limited and high detection performance isobtained.

In an acceleration sensor of the fifteenth aspect of the presentinvention, the vibrator and weight section are bonded together with anadhesive, and the groove is filled with a filler whose Young's modulusis smaller than that of the adhesive. If the inside of the groove is ina hollow state, certainly sliding vibration is not limited. However, theprocess of producing the hollow state is not easy by any means, and itis also hard to say that the process of connecting the vibrator andweight section with an adhesive while maintaining the hollow state iseasy. Even when the groove is filled with some filler, if the hardnessof the filler is low, sliding vibration is hardly limited. Therefore,the groove is filled with a filler whose Young's modulus is smaller thanthat of an adhesive used for connecting the vibrator and weight section.In this case, it is possible to minimize the deterioration of thedetection performance and readily perform the process of connecting thevibrator and weight section with an adhesive.

In an acceleration sensor of the sixteenth aspect of the presentinvention, the groove has a width greater than the widths of the divideddetection portions of the vibrator. Since the width of the groove of thevibrator is made greater than the widths of the respective portionswhere the electrodes are formed, the bonded area other than theelectrodes is increased, thereby improving the resistance to shock.

In an acceleration sensor of the seventeenth aspect of the presentinvention, the vibrator and the weight section are bonded together withan adhesive, and the area of this bonded face is greater than the areaof the electrode and/or the wiring pattern. Since the area of theelectrode and/or the wiring pattern is made smaller than the bonded areabetween the vibrator and weight section, even when the electrode and/orthe wiring pattern do not easily stick to the adhesive, the vibrator andweight section can be bonded at portions other than the electrode andwiring pattern, thereby achieving a high bonding strength.

In an acceleration sensor of the eighteenth aspect of the presentinvention, the weight section has a cavity for storing a part of thevibrator. Since a region of the weight section connected to the vibratorhas a cavity structure, even when its portion connected to the vibratoris made thinner so as to improve the detection sensitivity, the weightsection is not twisted by an applied acceleration, thereby limiting anincrease of crosstalk and improving the resistance to shock.

In an acceleration sensor of the nineteenth aspect of the presentinvention, the vibrator is a single crystal piezoelectric element. Sincea single crystal piezoelectric element is used for the vibrator, it ispossible to reduce crosstalk and improve the S/N ratio.

In an acceleration sensor of the twentieth aspect of the presentinvention, the single crystal piezoelectric element is an X-cut plate oflithium niobate (LiNbO₃). Since the X-cut plate of LiNbO₃ which does nothave an electromechanical bonding in a thickness direction is used asthe vibrator, it is possible to reduce crosstalk and improve the S/Nratio.

An acceleration sensor device of the twenty first aspect of the presentinvention comprises an acceleration sensor as described above; and apackage, including a base provided with a wiring pattern and a capcovering the base, for storing the acceleration sensor therein. Sincethe acceleration sensor is stored in the package including the baseprovided with the wiring pattern and the cap, it is possible to readilyperform the process of storing the acceleration sensor and to easilyconstruct a circuit for drawing an electrical signal.

An acceleration sensor device of the twenty second aspect of the presentinvention includes in a package a detection circuit for detecting anacceleration based on an output electrical signal. Since the detectioncircuit is stored in the package together with an acceleration sensor,it is possible to improve the S/N ratio.

In an acceleration sensor device of the twenty third aspect of thepresent invention, a part of the weight section is sandwiched bypackage. It is therefore possible to readily provide an electricalconnection between the weight section and package and to reduce the sizeof the device.

In an acceleration sensor device of the twenty fourth aspect of thepresent invention, a part of the weight section and a part of thevibrator are sandwiched by the package. It is therefore possible toreadily provide an electrical connection between the weight section andpackage and to reduce the size of the device.

In an acceleration sensor device of the twenty fifth aspect of thepresent invention, the part of the weight section sandwiched by thepackage is a portion located on one side of the position of the vibratoropposite to the other side including the position of the center ofgravity. Since the vibrator and the portion of the weight sectionlocated on one side of the position of the vibrator opposite to theother side, including the position of the center of gravity aresandwiched by the package, even though they are sandwiched by thepackage, the sandwiched portions are apart from the position of thecenter of gravity on which an acceleration applies. Therefore, thedetection sensitivity is not lowered.

In an acceleration sensor device of the twenty sixth aspect of thepresent invention, the length of the part of the weight sectionsandwiched by the package is not more than the length of the vibrator.It is therefore possible to improve the detection sensitivity.

In an acceleration sensor device of the twenty seventh aspect of thepresent invention, the vibrator has a package-side electrode, thepackage is provided with a wiring pattern connected to the electrode,the vibrator and package are bonded together with an adhesive, and thearea of the bonded face is larger than the area of the electrode and/orthe wiring pattern. Therefore, even when the electrode and/or the wiringpattern do not easily stick to the adhesive, the vibrator and packageare bonded together at portions other than the electrode and wiringpattern, thereby achieving a high bonding strength.

A method of fabricating an acceleration sensor of the twenty eighthaspect of the present invention comprises the steps of forming a groovein a vibrator; applying a material produced by dissolving a powder in avolatile solvent to the groove; evaporating the volatile solvent; andconnecting the vibrator and a weight section with an adhesive. Thepowder layer functions as a cap to prevent the adhesive from enteringthe groove, thereby maintaining the inside of the groove in a hollowstate.

A method of fabricating an acceleration sensor of the twenty ninthaspect of the present invention comprises the steps of forming a groovein a vibrator; inserting a structural body into the groove; connectingthe vibrator and weight section with an adhesive; and removing thestructural body. This method can prevent the adhesive from entering thegroove and maintain the groove in a hollow state.

The above and further objects and features of the invention will morefully be apparent from the following detailed description withaccompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is an explanatory view showing the detection principle of anacceleration sensor of the present invention;

FIG. 2 is a perspective view showing the structure of an accelerationsensor according to the first embodiment;

FIG. 3 is a perspective view showing a vibrator of the accelerationsensor according to the first embodiment;

FIG. 4 is a perspective view showing a weight section of theacceleration sensor according to the first embodiment;

FIG. 5 is a depiction showing a process of bonding the vibrator andweight section and the vibrator and a specimen;

FIG. 6 is an illustration showing one example of a detection circuit ofan acceleration sensor of the present invention;

FIG. 7 is a perspective view showing the structure of an accelerationsensor according to the second embodiment;

FIG. 8 is a perspective view showing a vibrator of the accelerationsensor according to the second embodiment;

FIG. 9 is a graph showing the relationship between the standardizedoutput voltage and the ratio D/T₁ of the depth D of the groove to thethickness T₁ of the vibrator;

FIG. 10 is a graph showing the relationship between the position in alongitudinal direction of the vibrator and the displacement in anexcitatory direction of the vibrator;

FIG. 11 is a graph showing the relationship between crosstalk and theratio W₁/W₂ of the width W₁ of the vibrator to the width W₂ of theweight section;

FIG. 12 is a perspective view showing the structure of an accelerationsensor according to the fifth embodiment;

FIG. 13 is a perspective view showing the structure of an accelerationsensor according to the first example of the sixth embodiment;

FIG. 14A is a perspective view of a weight section of an accelerationsensor according to the second example of the sixth embodiment;

FIG. 14B is a cross sectional view of the weight section of theacceleration sensor according to the second example of the sixthembodiment;

FIG. 15A is a perspective view of a weight section of an accelerationsensor according to the third example of the sixth embodiment;

FIG. 15B is a cross sectional view of the weight section of theacceleration sensor according to the third example of the sixthembodiment;

FIG. 16A is a perspective view of a weight section of an accelerationsensor according to the fourth example of the sixth embodiment;

FIG. 16B is a cross sectional view of the weight section of theacceleration sensor according to the fourth example of the sixthembodiment;

FIG. 17A is a perspective view of a weight section of an accelerationsensor according to the fifth example of the sixth embodiment;

FIG. 17B is a cross sectional view of the weight section of theacceleration sensor according to the fifth example of the sixthembodiment;

FIG. 18 is a depiction showing a process of connecting the weightsection and vibrator and connecting the vibrator and specimen accordingto the first example of the seventh embodiment;

FIG. 19 is a depiction showing a process of connecting the weightsection and vibrator and connecting the vibrator and specimen accordingto the second example of the seventh embodiment;

FIG. 20 is a perspective view showing the structure of an accelerationsensor according to the eighth embodiment;

FIG. 21 is an illustration showing the connected state of anacceleration sensor and a detection circuit substrate according to thefirst example of the ninth embodiment;

FIG. 22 is an illustration showing the connected state of anacceleration sensor and a detection circuit substrate according to thesecond example of the ninth embodiment;

FIGS. 23A through 23C are depictions showing a process of fabricating anacceleration sensor of the present invention;

FIG. 24 is a perspective view showing the structure of an accelerationsensor according to the tenth embodiment;

FIG. 25 is a perspective view of a vibrator of the acceleration sensoraccording to the tenth embodiment;

FIG. 26 is a perspective view of a weight section of the accelerationsensor according to the tenth embodiment;

FIGS. 27A and 27B are illustrations showing one example of a detectioncircuit of the acceleration sensor according to the tenth embodiment:

FIG. 28 is a graph showing the relationship between the divided positionand standardized output voltage and crosstalk in detection by adifferential output and by a single output;

FIG. 29 is a perspective view showing the structure of an accelerationsensor according to the first example of the eleventh embodiment;

FIG. 30 is a perspective view showing the structure of an accelerationsensor according to the second example of the eleventh embodiment;

FIG. 31 is a perspective view showing the structure of an accelerationsensor according to the twelfth embodiment;

FIG. 32A is a front view of a weight section of the acceleration sensoraccording to the twelfth embodiment;

FIG. 32B is a cross sectional view of the weight section of theacceleration sensor according to the twelfth embodiment;

FIG. 32C is a rear view of the weight section of the acceleration sensoraccording to the twelfth embodiment;

FIGS. 33A through 33C are cross sectional views showing a process offabricating an acceleration sensor according to the thirteenthembodiment;

FIG. 34 is a cross sectional view showing the bonded state of a vibratorand a weight section of an acceleration sensor according to thefourteenth embodiment;

FIGS. 35A through 35C are cross sectional views showing a process offabricating an acceleration sensor according to the fifteenthembodiment;

FIGS. 36A through 36D are perspective views and cross sectional viewsshowing a process of fabricating an acceleration sensor according to thesixteenth embodiment;

FIGS. 37A and 37B are perspective views showing other examples of thesixteenth embodiment;

FIG. 38 is a cross sectional view showing the bonded state of a vibratorand a weight section of an acceleration sensor according to theseventeenth embodiment;

FIGS. 39A and 39B are cross sectional views showing the bonded state ofa vibrator and a weight section of an acceleration sensor according tothe eighteenth embodiment;

FIG. 40A is a plan view of the weight-section side of a vibrator of anacceleration sensor according to the nineteenth embodiment;

FIG. 40B is a plan view of the vibrator side of a weight section of theacceleration sensor according to the nineteenth embodiment;

FIGS. 41A and 41B are plan views of the weight-section side of thevibrator of the acceleration sensor according to the nineteenthembodiment;

FIGS. 41C and 41D are plan views of the vibrator side of the weightsection of the acceleration sensor according to the nineteenthembodiment;

FIG. 42A is a cross sectional view showing the structure of anacceleration sensor according to the first example of the twentiethembodiment;

FIG. 42B is a bottom view showing the structure of the accelerationsensor according to the first example of the twentieth embodiment;

FIG. 43 is a cross sectional view showing the structure of anacceleration sensor according to the second example of the twentiethembodiment;

FIG. 44 is a cross sectional view showing the structure of anacceleration sensor device according to the twenty first embodiment;

FIG. 45 is a cross sectional view showing the structure of anacceleration sensor device according to the twenty second embodiment;

FIG. 46A is a cross sectional view of a base section of a package of anacceleration sensor device according to the twenty third embodiment;

FIG. 46B is a bottom view of the base section of the package of theacceleration sensor device according to the twenty third embodiment;

FIG. 47A is a cross sectional view of a cap section of the package ofthe acceleration sensor device according to the twenty third embodiment;

FIG. 47B is a bottom view of the cap section of the package of theacceleration sensor device according to the twenty third embodiment;

FIG. 48 is a cross sectional view showing the structure of theacceleration sensor device according to the twenty third embodiment;

FIG. 49 is a cross sectional view showing the structure of anacceleration sensor device according to the first example of the twentyfourth embodiment;

FIG. 50 is a cross sectional view showing the structure of anacceleration sensor device according to the second example of the twentyfourth embodiment;

FIG. 51A is a plan view of a weight section of an acceleration sensordevice according to the twenty fourth embodiment;

FIG. 51B is a cross sectional view of the weight section of theacceleration sensor device according to the twenty fourth embodiment;

FIG. 52 is a graph showing the relationship between the standardizedoutput voltage and the ratio W₂₁/W₂ of the width W₂₁ of an extended endportion to the entire width W₂ of the weight section;

FIG. 53 is a cross sectional view showing the structure of anacceleration sensor device according to the twenty fifth embodiment;

FIG. 54 is a graph showing the relationship between the standardizedoutput voltage and the ratio L₃/L₁ of the length L₃ of a sandwichedportion to the entire length L₁ of the vibrator;

FIG. 55 is a cross sectional view showing the structure of anacceleration sensor device according to the first example of the twentysixth embodiment;

FIG. 56 is a cross sectional view showing the structure of anacceleration sensor device according to the second example of the twentysixth embodiment;

FIG. 57A is a plan view of the package side of a vibrator of anacceleration sensor device according to the twenty seventh embodiment;

FIG. 57B is a plan view of the vibrator side of a base section of thepackage of the acceleration sensor device according to the twentyseventh embodiment;

FIGS. 58A and 58B are plan views of the vibrator side of the basesection of the package of the acceleration sensor device according tothe twenty seventh embodiment; and

FIG. 59 is a cross sectional view showing the structure of anacceleration sensor device according to the twenty eighth embodiment.

The sliding vibration can be exemplified as a shearing strain, that is,a pair of forces equal and opposite which cause a shearing force insidea detector. This is explained below.

DETAILED DESCRIPTION OF THE INVENTION

The following description will explain the present invention withreference to the drawings illustrating embodiments thereof.

First, the detection principle of an acceleration sensor of the presentinvention will be explained. FIG. 1 is an explanatory view illustratingthe detection principle. An acceleration sensor of the present inventionincludes a vibrator 100, a weight section 200 and detection section 300respectively connected to the vibrator 100. The weight section 200 issupported at a support point S, and the position of this support point Sis different from the position of the center of gravity G of an assemblyof the vibrator 100 and weight section 200. When an acceleration in onedirection (the direction of a void arrow in FIG. 1) is applied to suchan acceleration sensor, an angular moment about the support point S(arrow A, size MLa in FIG. 1 (where M is the mass of the weight section200, L is the length from the support point S to the center of gravityof the weight section 200, and a is the applied acceleration)) isexerted. This angular moment causes sliding vibration of the vibrator100 (arrow B in FIG. 1). The detection section 300 detects a signalresulting from sliding vibration corresponding to such an angularmoment. Since the size of the angular moment is proportional to the sizeof an acceleration to be detected, it is possible to detect theacceleration by detecting this signal.

Next, embodiments of the acceleration sensor of the present inventionwill be explained.

(First Embodiment)

FIG. 2 is a perspective view showing the structure of an accelerationsensor 10 according to the first embodiment of the present invention,FIG. 3 is a perspective view of a vibrator 1 of the acceleration sensor10, and FIG. 4 is a perspective view of a weight section 2 of theacceleration sensor 10. The acceleration sensor 10 includes therectangular parallelepiped vibrator 1 (length L₁: 1.3 mm, width W₁: 2.5mm, thickness T₁: 0.5 mm) formed of, for example, a LiNbO₃ singlecrystal piezoelectric element with 165° Y and θ=39°, and the rectangularparallelepiped weight section 2 (length L₂: 5.8 mm, width W₂: 2.5 mm,thickness T₂: 0.5 mm, density p: about 7.5×10³ kg/m³) made fromaluminum.

As shown in FIG. 3, electrodes 1 a, 1 a are formed by longitudinallydividing an electrode into substantially two equal parts are provided onthe front face (a face to be bonded to the weight section 2) of thevibrator 1. Moreover, as shown in FIG. 4, wiring patterns 2 a, 2 a areprovided on the rear face of the weight section 2, at positions facingthe electrodes 1 a, 1 a, in such a manner that the wiring patterns 2 a,2 a extend to a region of the front face of the weight section 2.

The vibrator 1 is connected to a supported end portion of the weightsection 2 in such a manner that the electrodes 1 a, 1 a and the wiringpatterns 2 a, 2 a are bonded to face each other. FIG. 5 is a depictionshowing this bonding process. The electrodes 1 a, 1 a and wiringpatterns 2 a, 2 a are bonded together with an adhesive 3 so as to form aconducting path, and, as shown in FIG. 5, the rear face of the vibrator1 is bonded to a specimen 4 to be detected an acceleration with theadhesive 3 when detecting an acceleration. In order to facilitate theelectrical connection between the vibrator 1 and weight section 2 andbetween the vibrator 1 and specimen 4, a conductive adhesive is used asthe adhesive 3.

A preferred conductive adhesive is an anisotropic conductive adhesive.The anisotropic conductive adhesive has a conducting property only inone direction, and retains an insulating property in other direction.Therefore, this anisotropic conductive adhesive is used for theacceleration sensor 10 of the present invention so that the conductiveproperty is exhibited only in the thickness direction (the verticaldirection in FIG. 5). Accordingly, it is possible to prevent a shortcircuit connection between the divided electrodes 1 a, 1 a of thevibrator 1 and to ensure and facilitate the electrical connectionbetween the divided electrodes 1 a, 1 a and the corresponding wiringpatterns 2 a, 2 a, thereby improving the reliability of the electricalconnection. Moreover, since the anisotropic conductive adhesive can beuniformly applied onto the divided electrodes 1 a, 1 a of the vibrator1, it is possible to improve the readiness of fabrication and thebonding strength.

Next, the following description will explain an operation of detectingan acceleration in the acceleration sensor 10 having such a structure.FIG. 6 is an illustration showing one example of a detection circuit.There is provided a differential amplifier 5 whose input terminals areconnected to the electrodes 1 a, 1 a through the wiring patterns 2 a, 2a (not shown in FIG. 6).

In the acceleration sensor 10 bonded to the specimen 4, when anacceleration in one direction (the widthwise direction, the void arrowdirection in FIG. 2) is applied, an angular moment about the supportpoint is exerted in the weight section 2 by the positional differencebetween the center of gravity of the weight section 2 and the supportpoint, and sliding vibrations (shearing strains) of differentorientation in the widthwise direction are exerted in both of thedivided regions of the vibrator 1. Then, by drawing a voltage resultingfrom this sliding vibration (shearing strain) from both of theelectrodes 1 a, 1 a, amplifying and detecting the potential differencethrough a differential amplifier 5, an acceleration is detected. Theshearing forces on the electrodes 1 a, 1 a causes shearing strains inthem, which results in voltages to measure the acceleration.

In such an acceleration sensor, since the weight section 2 alsofunctions as a signal detecting substrate, it is possible to decreasethe cost. Moreover, although the electrodes 1 a, 1 a of the vibrator 1are hidden by the weight section 2, since the wiring patterns 2 a, 2 aconnected to the electrodes 1 a, 1 a are extended to the front face ofthe weight section 2, the detection signals are easily drawn.

For such an acceleration sensor 10, it is possible to fabricate a numberof vibrators 1 at a time. Specifically, after forming a pattern of twolines of electrodes on one piece of piezoelectric single crystal wafer,a plurality of vibrators 1 can be readily cut out from the wafer by, forexample, dicing. Incidentally, as a method of forming the electrodepattern, it is possible to use, for example, screen printing,sputtering, plating and etching. As described above, in the firstembodiment, the fabrication process is easy and a significant reductionin the cost is achievable.

Further, the above-mentioned example illustrates a case where a facehaving the divided electrodes 1 a, 1 a is located on the front face side(the weight section 2 side). However, even if the face having thedivided electrodes 1 a, 1 a is located on the opposite side, i.e., therear face side, of course the acceleration sensor 10 has the samefunction and effect and exhibits the same detection characteristics asabove.

When an acceleration is applied in a direction (the thickness directionor longitudinal direction of the vibrator 1) other than the direction ofan acceleration to be detected (the widthwise direction of thevibrator), an unnecessary output voltage is sometimes generateddepending on the degree of distortion of the vibrator 1. This voltage iscrosstalk and a major cause of deterioration of the S/N ratio.

In order to improve the S/N ratio by reducing crosstalk, it ispreferable to use an X-cut plate of LiNbO₃ as the piezoelectric singlecrystal of the vibrator 1. For example, when an acceleration other thanthe one to be detected is applied in the thickness direction of thevibrator 1, the major cause of crosstalk is charge produced by anelectromechanical coupling in the thickness direction. Since the X-cutplate of LiNbO₃ has no electromechanical coupling in the thicknessdirection, if it is used for the acceleration sensor 10 of the presentinvention which detects an acceleration in the widthwise direction, itis possible to reduce crosstalk significantly and achieve a satisfactoryS/N ratio.

(Second Embodiment)

FIG. 7 is a perspective view showing the structure of an accelerationsensor 10 according to the second embodiment of the present invention,and FIG. 8 is a perspective view of the vibrator 1 of the accelerationsensor 10. In the second embodiment, dividing of the detection portionis ensured by not only the divided electrodes 1 a, 1 a, but also agroove 1 b (depth D: 0.05 mm) formed in the vibrator 1. Except for theformation of the groove 1 b, the vibrator 1 of the second embodiment isthe same in other configurations and material as that of the firstembodiment. Besides, the weight section 2 is the same as that of thefirst embodiment.

Even for such an acceleration sensor 10, it is possible to fabricate anumber of the vibrators 1 at a time. Specifically after forming apattern of electrodes over the entire front face of one piece ofpiezoelectric single crystal wafer by screen printing, sputtering,plating or other method, it is possible to easily fabricate a pluralityof vibrators 1 provided with the electrodes 1 a, 1 a and groove 1 b by,for example, dicing, etching, sandblasting, or other method. Here, byprogramming the cutting of each vibrator 1 and the dividing of thedetection portion at the beginning, a number of the vibrators 1 can becompletely automatically fabricated, thereby achieving a significantreduction in the cost.

Incidentally, the above-mentioned example illustrates a case where thegroove 1 b is formed in the front face (the weight section 2 side),However, even if the groove 1 b is formed in the opposite side, i.e.,the rear face, of course the acceleration sensor 10 has the samefunction and effect and exhibits the same detection characteristics asabove.

Next, the relationship between the depth D of the groove 1 b and thethickness T₁ of the vibrator 1 will be explained. FIG. 9 is a graphshowing the relationship between the ratio D/T₁ of the depth D of thegroove 1 b to the thickness T₁ of the vibrator 1 (the horizontal axis)and the standardized output voltage (logarithmic scale, the verticalaxis). Here, of course, this output voltage is equivalent to thedetection sensitivity. It will be appreciated from the results shown inFIG. 9 that the output voltage increases abruptly until the ratio D/T₁reaches “0.1”, and then gradually rises with an increase of the ratioD/T₁. Therefore, in order to obtain a sufficient detection sensitivity,it is necessary to provide the groove 1 b having a depth D that givesthe ratio D/T₁ of not less than 0.1.

From the view point of the detection sensitivity, it is most effectiveto provide two piezoelectric vibrators side by side. In this case,however, there is a problem that the fabrication cost increases.

In the second embodiment, by limiting the depth D of the groove 1 b asdescribed above (to a value not less than 0.1 times the thickness T₁ ofthe vibrator 1), it is possible to hold a high detection sensitivitysimilar to that obtained by the two piezoelectric vibrators providedside by side, thereby achieving both of a reduction in the cost and animprovement in the sensitivity.

(Third Embodiment)

Next, the divided position of the detection portion in the first andsecond embodiments will be reviewed. In the acceleration sensor 10 ofthe present invention, the sliding directions in the two detectionportions are opposite to each other between both sides of the supportpoint (the center of rotation) of the weight section 2. In other words,the generated charges and in turn the output voltages have oppositepolarities between the both sides of that point.

FIG. 10 is a graph showing the relationship between the position in alongitudinal direction of the vibrator 1 (the horizontal axis) and thedisplacement in an excitatory direction of the vibrator 1 (the verticalaxis) according to the result of a simulation performed by applying anacceleration in a widthwise direction of the acceleration sensor 10. Thevibrator 1 used in this simulation satisfied the conditions that it wasformed from LiNbO₃ with 165° Y and θ=39° to have 1.3 mm in length L₁,1.0 mm in width W₁ and 0.5 mm in thickness T₁, while one similar to theweight section 2 of the second embodiment was used as the weight section2. Incidentally, a value “0” in the horizontal axis of FIG. 10represents the center in the longitudinal direction, and positions inthe longitudinal direction are shown by indicating a directionapproaching the center of gravity of the weight section 2 as positive.

It will be appreciated from the result shown in FIG. 10 that thedisplacements on the front face of the vibrator 1 are not symmetricalabout the center (the “0” position) in the longitudinal direction of thevibrator 1. Therefore, when the detection portion is divided at thecenter in the longitudinal direction of the vibrator 1, the generatedcharge is canceled and consequently the output voltage is lowered in thenegative region in the longitudinal direction. Hence, when the detectionportion is divided along a line crossing a point where the displacementis “0”, i.e., a point where the angular moment of the weight section 2is “0”, and similarly a point where the generated charge is “0”,opposite output voltages are efficiently obtained. As a result, thedetection sensitivity is improved.

In the example shown in FIG. 10, since the position in the longitudinaldirection where the displacement is “0” is −0.2 mm, it is possible toachieve a high detection sensitivity by dividing the detection portionat this position (the position 0.2 mm off from the center in thelongitudinal direction toward the center of gravity of the weightsection 2). Incidentally, the value indicating this divided position isone example, and of course the divided position of the vibrator 1 varieswhen the conditions of the vibrator 1 and weight section 2 are changed.

In the third embodiment, as described above, since the detection portionof the vibrator 1 is divided at a position where the charge distributionis substantially zero, i.e., a position where the angular moment issubstantially zero, it is possible to hold a high detection sensitivityand achieve both of a reduction in the cost and an improvement in thesensitivity.

(Fourth Embodiment)

Next, the relationship between the width W₁ of the vibrator 1 and thewidth W₂ of the weight section 2 of the acceleration sensor 10 of thepresent invention will be reviewed.

FIG. 11 is a graph showing the relationship between the ratio W₁/W₂ ofthe width W₁ of the vibrator 1 to the width W₂ of the weight section 2(the horizontal axis) and crosstalk (the vertical direction) accordingto the result of a simulation performed by applying an acceleration in athickness direction which is not a direction of an acceleration to bedetected by the acceleration sensor 10. The vibrator 1 used in thissimulation satisfied the conditions that it was formed from LiNbO₃ with165° Y and θ=20° to have 1.3 mm in length L₁ and 0.5 mm in thickness T₁,while one similar to the weight section 2 of the first and secondembodiment was used as the weight section 2, and the ratio W₁/W₂ wasvaried by changing the width W₁ of the vibrator 1.

It will be appreciated from the result shown in FIG. 11 that crosstalkincreases as the ratio W₁/W₂ increases, and particularly, crosstalkshows an abrupt increase when the ratio W₁/W₂ exceeds “1”, i.e., whenthe width W₁ of the vibrator 1 is larger than the width W₂ of the weightsection 2. Thus, if the width W₁ of the vibrator 1 is made larger thanthe width W₂ of the weight section 2, crosstalk becomes greater than adesired detection signal, and consequently an accurate acceleration inthe widthwise direction cannot be detected. Therefore, in theacceleration sensor 10 of the fourth embodiment, the width W₁ of thevibrator 1 is set less than the width W₂ of the weight section 2 so asto reduce crosstalk and achieve a satisfactory S/N ratio.

(Fifth Embodiment)

In the acceleration sensor 10 of the present invention, as the lengthfrom the support point of the weight section 2 to the center of gravitythereof is increased and as the mass of the weight section 2 isincreased, the angular moment of the weight section 2 becomes larger andthe detection sensitivity is improved. FIG. 12 is a perspective viewshowing the structure of an acceleration sensor 10 according to thefifth embodiment of the present invention;

In this acceleration sensor 10, the thickness of the weight section 2 isnot uniform, and the thickness (T_(2A): 0.5 mm) of a substantially halfportion on the vibrator 1 side (the supported-end side) is thinner thanthe thickness (T_(2B): 1.5 mm) of the remaining substantially halfportion on the opposite side (the free-end side). Thus, by making thethickness of the free-end side portion thicker than that of thesupported-end side portion so as to increase the length from the supportpoint to the center of gravity and to increase the entire mass, thedetection sensitivity is improved.

Incidentally, in the example shown in FIG. 12, the thickness isincreased in both of the directions to the front side and rear side.However, needless to say, it is possible to increase the thickness ineither of the directions to the front side and rear side.

Besides, the above example illustrates the case where the thickness ofthe free-end side portion of the weight section 2 is made thicker.However, even when a weight section 2 in which the width of the free-endside portion is larger than that of the supported-end side portion isused, of course a similar effect is obtained.

(Sixth Embodiment)

In the acceleration sensor 10 of the present invention, as alsoexplained in the fifth embodiment, by increasing the length from thesupport point of the weight section 2 to the center of gravity thereofand by increasing the mass of the weight section 2, it is possible toincrease the angular moment of the weight section 2 that is proportionalto the product of the length and mass and to improve the detectionsensitivity. Accordingly, in the sixth embodiment, the weight section 2is constructed by using a plurality of materials of different densitiesso that a portion on the vibrator 1 side (supported-end side portion) isformed from a material with a low density and the opposite portion(free-end side portion) is formed from a material with a high density.For example, it is possible to form the supported-end side portion fromalumina (the density: about 7.5×10³ kg/m³) whereby the wiring patterns 2a, 2 a are easily formed, and to form the free-end side portion from amaterial such as molybdenum (the density: about 1.9×10⁴ kg/m³), tungsten(the density: about 1.0×10⁴ kg/m³)or the like which have a high densityand are easily molded.

(Sixth Embodiment: First Example)

FIG. 13 is a perspective view showing the structure of an accelerationsensor 10 according to the first example of the sixth embodiment. Theweight section 2 of this acceleration sensor 10 is constructed by afirst weight 21, which is made from alumina and also functions as asignal detecting substrate like the first and second embodiments, andsecond weights 22, 22 which are made from molybdenum and attached to thefront and rear faces of the free-end side portion of the first weight21.

By constructing the weight section 2 in this manner, the density (ρ_(B))of the free-end side portion is greater than the density (ρ_(A)) of thesupported-end side portion (ρ_(B)>ρ_(A)) and the angular moment of theweight section 2 becomes larger, thereby further improving the detectionsensitivity.

Incidentally, in the example shown in FIG. 13, the second weight 22 isattached to each of the front and rear faces of the first weight 21.However, needless to say, the second weight 22 may be provided only onthe front face or rear face. Moreover, in the example shown in FIG. 13,although the first weight 21 and the second weight 22 have the samewidth, it is possible to make the width of the second weight 22 largerthan that of the first weight 21 so as to further increase the angularmoment.

(Sixth Embodiment: Second Example and Third Example)

FIGS. 14A and 14B are the perspective view and cross sectional view ofthe weight section 2 of an acceleration sensor 10 according to thesecond example of the sixth embodiment, and FIGS. 15A and 15B are theperspective view and cross sectional view of the weight section 2 of anacceleration sensor 10 according to the third example of the sixthembodiment. The second and third examples are constructed so as to makethe angular moment of the weight section 2 greater than that of thefirst example.

In the second example, the weight section 2 is constructed by providingthe second weight 22 on the front face of the free-end side portion ofthe first weight 21 in such a manner that the second weight 22 is partlyburied in a groove portion 21 a of the first weight 21. Incidentally, inthe example shown in FIGS. 14A and 14B, while the second weight 22 isprovided on only one of the faces of the first weight 21, it may beprovided on both the faces. In the third example, the weight section 2is constructed by fitting the second weight 22 into a hole 21 b formedin the free-end side portion of the first weight 21.

The second example in which the second weight 22 is buried in the firstweight 21 and the third example in which the second weight 22 is fittedinto the first weight 21 provide excellent resistance to shock and aresuitable for use in vehicles having a high possibility to get strongshock.

(Sixth Embodiment: Fourth Example and Fifth Example)

FIGS. 16A and 16B are the perspective view and cross sectional view ofthe weight section 2 of an acceleration sensor 10 according to thefourth example of the sixth embodiment, and FIGS. 17A and 17B are theperspective view and cross sectional view of the weight section 2 of anacceleration sensor 10 according to the fifth example of the sixthembodiment. The weight section 2 of each of the fourth and fifthembodiments is constructed by engaging the first weight 21 on thesupported-end side with the weight 22 on the free-end side. In thefourth example, the engaged region has a two-layer structure consistingof the second weight 22 on the front-face side and the first weight 21on the rear-face side. In the fifth example, the engaged region has athree-layer structure consisting of the first weight 21 on thefront-face side and on the rear-face side, and the second weight 22 inthe middle.

In the fourth and fifth examples, if the acceleration sensor 10 isconstructed so that the center of gravity of the first weight 21 islocated on the vibrator 1, there is no possibility that the weightsection 21 hangs down due to its own weight in bonding the first weight21 to the vibrator 1 with an anisotropic conductive adhesive or thelike, thereby facilitating the bonding process. Besides, if theacceleration sensor 10 is constructed so that the center of gravity ofthe second weight 22 is positioned within the engaged region, there isno possibility that the second weight 22 hangs down due to its ownweight in bonding the second weight 22 to the first weight 21 with anadhesive, etc., thereby facilitating the bonding process.

(Seventh Embodiment: First Example and Second Example)

FIG. 18 is a depiction showing a process of connecting the weightsection 2 and vibrator 1 and connecting the vibrator 1 and specimen 4according to the first example of the seventh embodiment. In the firstexample, the weight section 2 and vibrator 1, and the vibrator 1 andspecimen 4 are electrically connected using bumps 31 of a solder, gold,etc. FIG. 19 is a depiction showing a process of connecting the weightsection 2 and vibrator 1 and connecting the vibrator 1 and specimen 4according to the second example of the seventh embodiment. In the secondexample, the weight section 2 and vibrator 1, and the vibrator 1 andspecimen 4 are electrically connected with a cream of solder 32. In suchfirst and second examples, it is possible to reduce a loss oftransmission energy at a resin portion generated by a resin-basedadhesive and to improve the readiness of fabrication and the detectionsensitivity.

(Eighth Embodiment)

FIG. 20 is a perspective view showing the structure of an accelerationsensor 10 according to the eighth embodiment. In the eighth embodiment,a FPC (flexible print connector) 33 is used to extend the signal linesfrom the wiring patterns 2 a, 2 a of the weight section 2 to a signaldetection circuit (not shown). With the use of the FPC 33, since thesignal lines are readily extended, it is possible to reduce the costsignificantly. Alternatively, it is possible to connect lines bybonding, such as a ribbon or a wire, instead of using the FPC 33.

(Ninth Embodiment: First Example and Second Example)

FIG. 21 is an illustration showing the connected state of anacceleration sensor 10 and a detection circuit substrate according tothe first example of the ninth embodiment. The wiring patterns 2 a, 2 aof the weight section 2 and the detection circuit substrate areconnected by wires 34, 34, and the connected portions are coated with aresin 35 such as silicone. FIG. 22 is an illustration showing theconnected state of an acceleration sensor 10 and a detection circuitsubstrate according to the second example of the ninth embodiment. Notonly the connected portions by wires 34, 34 are coated with the resin 35like the first example, but also the entire supported portion of theweight section 2 as well as the vibrator 1 is coated with the resin 35such as silicone.

In the ninth embodiment, such a resin coating is used to preventaccidents such as a disconnection of lines due to strong external shockand to improve the reliability. Besides, in the second example, it ispossible to prevent a separation between the weight section 2 andvibrator 1 and between the vibrator 1 and specimen 4 without limitingthe angular moment. Although the example using wire bonding has beenexplained, of course, this resin coating is also effective for the casesusing ribbon bonding or an FPC.

Here, the fabrication process of the acceleration sensor 10 of thepresent invention is briefly explained. FIGS. 23A through 23C aredepictions showing a process of fabricating a plurality of theacceleration sensors 10 of the present invention at a time. An aluminaplate 42 whose front and rear faces are provided with a plurality ofwiring patterns 42 a that are to be the wiring patterns 2 a of theweight section 2 is positioned on a long piezoelectric single crystalelement 41 whose front face is provided with a pattern of two lines ofelectrodes 41 a, 41 a that are to be the divided electrodes 1 a, 1 a ofthe vibrator 1 so that the wiring pattern 42 a faces the electrodes 41a, 41 a (FIG. 23A), and then the piezoelectric single crystal element 41and alumina plate 42 are bonded together with an anisotropic conductiveadhesive (FIG. 23B) Next, this bonded structure is cut into pieces ofsensors by dicing or other method to fabricate a plurality ofacceleration sensors 10 (FIG. 23C).

With such a fabrication process, it is possible to readily fabricate anumber of the acceleration sensors 10 at a time, decrease the number ofprocessing steps significantly, and achieve a reduction in the cost.

(Tenth Embodiment)

FIG. 24 is a perspective view showing the structure of an accelerationsensor 10 according to the tenth embodiment of the present invention,FIG. 25 is a perspective view of the vibrator 1 of the accelerationsensor 10, and FIG. 26 is a perspective view of the weight section 2 ofthe acceleration sensor 10. In the tenth embodiment, like the secondembodiment, the detection portion of the vibrator 1 is divided by notonly the divided electrodes 1 a, 1 a, but also the groove 1 b. Moreover,this divided position is not the center in a longitudinal direction ofthe vibrator 1 and is slightly off from the center to the free-end sideof the weight section 2, and the electrode 1 a on the free-end side isshorter in length than the electrode 1 a on the supported-end side.Further, the function and effect of this groove 1 b are the same asthose in the second embodiment.

In the above-described embodiments, the wiring patterns 2 a, 2 a areprovided at two positions on the weight section 2, facing the electrodes1 a, 1 a of the vibrator 1, respectively, while in the tenth embodiment,the wiring pattern 2 a is provided only at a position facing theelectrode 1 a on the free-end side in such a manner that it extends to aregion of the front face of the weight section 2, and thus the wiringpattern 2 a is not provided at a position facing the electrode 1 a onthe supported-end side.

In the above-described embodiments, an acceleration is detected based onthe difference between output voltages from both the electrodes 1 a, 1 a(the differential output). In the tenth embodiment, however, anacceleration is detected based only on an output voltage of theelectrode 1 a on the free-end side (single output). FIGS. 27A and 27Bare illustrations showing one example of a detection circuit accordingto the tenth embodiment. With the use of an FET 51 and a preamplifier52, the output voltage of the electrode 1 a on the free-end side isamplified and an acceleration is detected.

FIG. 28 is a graph showing the relationship between the divided position(the horizontal axis) and the standardized output voltage and crosstalk(the vertical axis) for each type of detection using the differentialoutput or single output, based on the results of simulations and actualmeasurements. The horizontal axis shows the standardized dividedpositions with “1” corresponding to the position of the free end of thevibrator 1, “0” the position of the supported end thereof and “0.5 ” thecenter in a longitudinal direction of the vibrator 1. The solid lines aand b indicate changes in the standardized output voltage in thedetection using the differential output and the detection using singleoutput, respectively. The broken lines c and d show changes in crosstalkin the detection using the differential output and the detection usingsingle output, respectively.  indicates the actually measured values ofthe standardized output voltage in the detection using single output.

In both of the detection using the differential output and the detectionusing single output, crosstalk is minimum when the detection portion ofthe vibrator 1 is divided into two equal parts (when the standardizeddivided position is “0.5”). It will be appreciated that, in the case ofthe detection using single output, as the divided position becomescloser to the position of the free end, the detected output voltage iscloser to that obtained in the detection using the differential outputand a similar detection sensitivity is obtained. For instance, with theuse of single output, in order to obtain a detection sensitivity similarto or higher than that obtained with the use of the differential outputwhen the vibrator 1 is equally divided, the vibrator 1 can be divided ata standardized divided position of not less than “0.65”. Moreover,considering crosstalk, when the value of crosstalk is limited to 5% orless, the vibrator 1 can be divided at a standardized divided positionof not more than “0.75”.

Incidentally, like the second embodiment, the above-described exampleillustrates the case where the groove 1 b is formed in the front face(the weight section 2 side). Similarly, when the groove 1 b is formed inthe opposite side (the rear face), needless to say, the detectioncharacteristics are the same as above. Furthermore, like the firstembodiment, even when the detection portion is divided by only theelectrodes 1 a, 1 a without forming the groove 1 b, of course, it ispossible to detect an acceleration by a single output from one electrode1 a on the free-end side.

As described above, in the tenth embodiment, since an acceleration canbe detected only by the output voltage from one of the electrodes 1 awith the same sensitivity as that in the detection based on thedifferential output, it is possible to decrease the circuit scale andsignificantly reduce the cost.

(Eleventh Embodiment: First Example and Second Example)

FIG. 29 is a perspective view showing the structure of an accelerationsensor 10 according to the first example of the eleventh embodiment ofthe present invention. Like the sixth embodiment, the weight section 2of the acceleration sensor 10 of the first example is constructed by afirst weight 21 made from alumina and second weights 22, 22 made frommolybdenum, One second weight 22 is mounted on the front face of thefirst weight 21, which also functions as a signal detecting substrate,so that it extends from the free end to a position slightly overlappingthe vibrator 1, while the other second weight 22 is mounted on the rearface of the first weight 21 so that it extends from the free end to aposition little before the vibrator 1. Alternatively, the second weight22 on the front face may have a length similar to that of the firstweight 21.

FIG. 30 is a perspective view showing the structure of an accelerationsensor according to the second example of the eleventh embodiment of thepresent invention. Like the sixth embodiment, the weight section 2 ofthe acceleration sensor 10 of the second example is constructed by afirst weight 21 made from alumina which also functions as a signaldetecting substrate and second weights 22, 22 made from molybdenum. Aportion of the first weight 21 from one end on the vibrator 1 to thecenter forms a thicker portion 21 c thicker than other portion, and onesecond weight 22 is provided on the front face of the first weight 21 sothat it extends from the free end and reaches the thicker portion 21 c,while the other second weight 22 is provided on the rear face of thefirst weight 21 so that it extends from the free end to a positionlittle before the vibrator 1. Alternatively, the thicker portion 21 c ofthe first weight 21 may be formed to extend over the entire area of thevibrator 1.

In both of the first and second examples, the thinnest portion of theweight section 2 is positioned on the vibrator 1. In a structure wherethe thinnest portion of the weight section 2 is located on one end sidewhich is not on the vibrator 1, when an acceleration in a thicknessdirection which is not the subject of detection is applied, resonanceoccurs at the thinnest portion and strong crosstalk results. Incontrast, in the eleventh embodiment, since the thinnest portion of theweight section 2 is supported and fixed on the vibrator 1, even if anacceleration in a thickness direction is applied, resonance does notoccur and crosstalk is little. It was confirmed by the experimentsperformed by the present inventors that, when an acceleration in athickness direction is applied, crosstalk of not less than 50% occurs inthe structure where the thinnest portion is not on the vibrator 1, butonly crosstalk of not more than 5% occurs in the structure of theeleventh embodiment where the thinnest portion is positioned on thevibrator 1.

Further, the above-described example illustrates the case where thethinnest portion of the weight section 2 is positioned on the vibrator1. However, since it is only preferred that this thinnest portion is notlocated on the free-end side from the vibrator 1, even if the weightsection 2 is further extended from the vibrator 1 to the supported-endside and the extended portion forms the thinnest portion, the samefunction and effect as those of the above-described examples areobtained.

(Twelfth Embodiment)

FIG. 31 is a perspective view showing the structure of an accelerationsensor 10 according to the twelfth embodiment of the present invention,FIG. 32A, FIG. 32B and FIG. 32C are the front view, cross sectional viewand rear view, respectively, of the weight section 2 of the accelerationsensor 10 according to the twelfth embodiment. In the weight section 2of the twelfth embodiment, wiring patterns 2 a, 2 a, 2 a, 2 a are formedon the front and rear faces at positions corresponding to the electrodes1 a, 1 a, and the wiring pattern 2 a on the front face and the wiringpattern 2 a on the rear face are electrically connected with athrough-hole 53. The wiring patterns 2 a, 2 a on the rear face and theelectrodes 1 a, 1 a are bonded together, so that the output voltagesfrom the electrodes 1 a, 1 a are drawn through the wiring patterns 2 a,2 a on the rear face, the through-hole 53 and the wiring patterns 2 a, 2a on the front face.

By the way, in the acceleration sensor of the preset invention asdescribed above, the detection portion for sliding vibration of thevibrator 1 is divided into a plurality of parts, a groove 1 b is formed,and the inside of this groove 1 b is made a hollow state. The reason forthis is that, if a relatively hard object is placed in this groove 1 b,reciprocal sliding vibration of the vibrator 1 to be exerted by anapplied acceleration is limited, and the detection performance islowered. As described above, the vibrator 1 and weight section 2 areconnected with an adhesive such as an anisotropic conductive adhesive soas to form a conducing path. In this case, however, there is apossibility that the adhesive runs into the groove 1 b and the groove 1b is filled with the adhesive. Therefore, in order to maintain theinside of the groove 1 b in a hollow state, it is necessary to take somemeasure. The following description will explain specific examples of themeasure (the thirteenth to sixteenth embodiments).

(Thirteenth Embodiment)

FIGS. 33A through 33C are cross sectional views showing a process offabricating an acceleration sensor according to the thirteenthembodiment. A material 81 produced by dissolving a powder in a volatilesolvent such as alcohol is rubbed into the groove 1 b formed in thevibrator 1, with a spatula or squeegee (FIG. 33A). As the powder, it ispossible to use a material having a relatively large particle diameter,such as Teflon powder and glass micro-balloon. Thereafter, by drying thematerial 81 to evaporate the solvent (such as alcohol), a powder layer82 is formed (FIG. 33B). Then, in order to electrically connect theelectrodes 1 a and wiring patterns 2 a, the vibrator 1 and weightsection 2 are bonded together with an adhesive 83 such an anisotropicconductive adhesive (FIG. 33C).

Since the powder layer 82 is formed on the top side (the weight section2 side) of the groove 1 b, it functions as a cap and prevents theadhesive 83 from running into the groove 1 b, thereby maintaining theinside of the groove 1 b in a hollow state. Accordingly, the slidingvibration of the vibrator 1 is not limited, and a lowering of thedetection performance does not occur.

(Fourteenth Embodiment)

FIG. 34 is a cross sectional view showing the bonded state of thevibrator and weight section of an acceleration sensor according to thefourteenth embodiment. In order to electrically connect the electrodes 1a and wiring patterns 2 a, the vibrator 1 and weight section 2 arebonded together with an anisotropic conductive film 84. Thus, since noadhesive is used, an adhesive can never run into the groove 1 b, theinside of the groove 1 b is maintained in a hollow state, the slidingvibration of the vibrator 1 is not limited, and a lowering of thedetection performance does not occur. Moreover, since the anisotropicconductive film 84 is used, it is possible to exhibit conductivity onlyin a thickness direction and ensure high insulation between theelectrodes 1 a, 1 a.

(Fifteenth Embodiment)

FIGS. 35A through 35C are cross sectional views showing a process offabricating an acceleration sensor according to the fifteenthembodiment. A dummy structural body 85 is inserted into the groove 1 bformed in the vibrator 1 (FIG. 35A). As the structural body 85, it ispossible to use a plate material, various kinds of resist material, waxmaterial and thermoplastic resin, such as Teflon, substantiallyidentical with the groove 1 b in shape. Next, in order to electricallyconnect the electrodes 1 a and wiring patterns 2 a, the vibrator 1 andweight section 2 are bonded together with the adhesive 83 such as ananisotropic conductive adhesive (FIG. 35B). Thereafter, the structuralbody 85 is removed (FIG. 35C).

In this example, it is also possible to prevent the adhesive 83 fromrunning into the groove 1 b and maintain the inside of the groove 1 b ina hollow state. Therefore, the sliding vibration of the vibrator 1 isnot limited, and a lowering of the detection performance does not occur.

(Sixteenth Embodiment)

FIGS. 36A through 36D are perspective views and cross sectional viewsshowing a process of fabricating an acceleration sensor according to thesixteenth embodiment. A protruding portion 86 having a width Ksubstantially equal to or larger than a width k of the groove 1 b isformed on a face of the weight section 2, to be connected to thevibrator 1, at a position corresponding to the groove 1 b (FIG. 36A). Asthe protruding portion 86, if it comes into contact with the electrodes1 a, 1 a, it is possible to use a structural body made from aninsulating material such as ceramics, resins, or the like. If theprotruding portion 86 does not come into contact with the electrodes 1a, 1 a, a conductive adhesive, metal, etc. may be used. The adhesive 83,such as epoxy adhesive and anisotropic conductive adhesive, is appliedto both sides of this protruding portion 86 (FIG. 36B). The weightsection 2 is placed over the vibrator 1 in such a manner that theprotruding portion 86 is positioned on the groove 1 b (FIG. 36C), andthe vibrator 1 and weight section 2 are bonded together (FIG. 36D) so asto electrically connect the electrodes 1 a and wiring patterns 2 a.

Since the protruding portion 86 is placed over the top side (the weightsection 2 side) of the groove 1 b, it functions as a cap, therebypreventing the adhesive 83 from running into the groove 1 b andmaintaining the inside of the groove 1 b in a hollow state. Therefore,sliding vibration of the vibrator 1 is not limited, and a lowering ofthe detection performance does not occur.

Here, although the protruding portion 86 has an angular shape, it mayhave other shape. FIGS. 37A and 37B are perspective views showing otherexamples of the sixteenth embodiment. FIG. 37A shows an example in whicha tapered protruding portion 86 is formed, while FIG. 37B illustrates anexample in which a rounded protruding portion 86 is formed.

Here, although the formation of the protruding portion 86 on the weightsection 2 has been explained, of course, it is possible to fabricate anacceleration sensor in the exactly the same manner by using a weightsection 2 whose portion facing the groove 1 b is made a protrudingportion in advance.

(Seventeenth Embodiment)

FIG. 38 is a cross sectional view showing the bonded state of thevibrator and weight section of an acceleration sensor according to theseventeenth embodiment. The vibrator 1 and weight section 2 are bondedtogether with the adhesive 83 such as an anisotropic conductive adhesivein such a manner that the electrodes 1 a and wiring patterns 2 a areelectrically connected, and the groove 1 b is filled with a filler 87such as a silicone resin whose Young's modulus is smaller than that ofthe adhesive 83.

If the inside of the groove 1 b is in a hollow state, it is certain thatthe sliding vibration is not limited. However, it is not always easy toperform the process of connecting the vibrator 1 and weight section 2with the adhesive 83 while maintaining this hollow state. Even when thegroove 1 b is filled with some filler, if the hardness of this filler islow, the sliding vibration is hardly limited. Therefore, the groove 1 bis filled with the filler 87 whose Young's modulus is smaller than thatof the adhesive 83 connecting the vibrator 1 and weight section 2. Insuch a case, the occurrence of deterioration of the detectionperformance can be minimized, and the process of connecting the vibrator1 and weight section 2 with the adhesive 83 is easily performed becausethe groove 1 b is filled with the filler 87.

(Eighteenth Embodiment)

FIGS. 39A and 39B are cross sectional views showing the bonded state ofthe vibrator and weight section of an acceleration sensor according tothe eighteenth embodiment. The eighteenth embodiment is an accelerationsensor suitable for use in such an environment subject to strong shock.The width k of the groove 1 b of the vibrator 1 is made wider than thewidths p₁ and p₂ of the respective portions where the electrodes 1 a, 1a are formed. Accordingly, since the bonded area between the vibrator 1and weight section 2 in addition to the bonded area between theelectrodes 1 a and wiring patterns 2 a is significantly increased, it ispossible to improve the resistance to shock. Additionally, in thisexample, the position of the groove 1 b and the widths of the respectiveportions may be made symmetrical (FIG. 39A) or asymmetrical (FIG. 39B).

(Nineteenth Embodiment)

FIGS. 40A and 40B are plan views of the weight section 2 side of thevibrator 1 and the vibrator 1 side of the weight section 2 of anacceleration sensor according to the nineteenth embodiment. An areaenclosed by the broken line in FIG. 40B is a region where the vibrator 1is to be bonded. The area of the electrodes 1 a of the vibrator 1 andthe wiring patterns 2 a of the weight section 2 is smaller than thebonded area between the vibrator 1 and weight section 2.

For instance, considering the problem of oxidation or corrosion, it issupposed to use gold as the material of the electrode 1 a and wiringpattern 2 a. In this case, there is a problem that the adhesive used forconnecting the vibrator 1 and weight section 2 does not easily stick togold, which would cause a lowering of the bonding strength, an increasein the transmission loss of energy which results in deterioration of thedetection performance, and a lowering of the resistance to shock.Therefore, in the nineteenth embodiment, the area of the electrodes 1 aand wiring patterns 2 a is made smaller than the bonded area so as toobtain a sufficient bonding strength in other region. Accordingly, evenwhen the electrodes 1 a and wiring patterns 2 a do not easily stick tothe adhesive, it is possible to increase the bonding strength betweenthe vibrator 1 and weight section 2.

Further, in the case where the area of the electrodes 1 a and wiringpatterns 2 a is smaller than the bonded area, the electrodes 1 a andwiring patterns 2 a may have an arbitrary shape. FIGS. 41A and 41B areplan views of the weight section 2 side of the vibrator 1 of theacceleration sensor according to the nineteenth embodiment, and FIGS.41C and 41D are plan views of the vibrator 1 side of the weight section2 of the same. In order to increase the degree of adhesion between thevibrator 1 and weight section 2, it is more preferable to arrange theelectrodes 1 a and wiring patterns 2 a so that they are symmetricalabout a line, symmetrical about a point and supported at three points.

(Twentieth Embodiment)

When the free-end side portion of the weight section 2 is made thicker(the fifth embodiment) or when the free-end side portion is formed froma high-dense material (the sixth embodiment) so as to improve thedetection sensitivity, the supported-end side portion of the weightsection 2 to be bonded to the vibrator 1 is made of a thin or low-densematerial. Therefore, there is a problem that the weight portion 2 may betwisted when an acceleration is applied and crosstalk may increase.Besides, since the supported-end side portion is thinner, the bondedarea between the free-end side portion and supported-end side portion ofthe weight section 2 is smaller, causing a problem that the resistanceto shock is low. The twentieth embodiment was devised to solve suchproblems.

(Twentieth Embodiment: First Example)

FIGS. 42A and 42B are the cross sectional view and bottom view showingthe structure of an acceleration sensor 10 according to the firstexample of the twentieth embodiment. The weight section 2 of thisacceleration sensor 10 is constructed by bonding a first weight 21 onthe supported-end side made from alumina to a second weight 22 on thefree-end side made from tungsten. A cavity 90 is formed on the lowerside of the first weight 21 on the supported-end side. The depth of thiscavity 90 is substantially the same as the depth of the division thatdivides the detection portion of the vibrator 1, one of the electrodes 1a of the vibrator 1 and the wiring pattern 2 a of the weight section 2formed on the cavity 90 are bonded together, and the upper side of thevibrator 1 is partly stored in the cavity 90.

As described above, since the cavity 90 is provided on the supported-endside of the weight section 2 to be bonded to the vibrator 1, even whenthe supported-end side portion is made thinner, it is reinforced by thewall of the cavity 90. Therefore, even when an acceleration is applied,the weight section 2 is not twisted, thereby enabling a reduction ofcrosstalk and accurate detection of the acceleration. Moreover, evenwhen the thickness of the portion of the weight section 2 to be bondedto the vibrator 1 is made thinner, since the bonded area between thefirst weight 21 and second weight 22 of the weight section 2 can beincreased, it is possible to improve the resistance to shock.Furthermore, since the free-end side portion of the weight 2 can be madethicker than the supported-end side portion thereof without increasingthe height of the acceleration sensor 10, it is possible to improve thedetection sensitivity without increasing the size of the structure.

(Twentieth Embodiment: Second Example)

FIG. 43 is a cross sectional view showing the structure of anacceleration sensor 10 according to the second example of the twentiethembodiment. The second example illustrate a case where the second weight22 on the free-end side is made thicker than the first weight 21 on thesupported-end side so as to further increase the angular moment of theweight section 2 compared with the first example and to further improvethe detection sensitivity.

Next, the following description will explain an embodiment of anacceleration sensor device constructed by storing an acceleration sensor10 of the present invention as described above in a package. Theacceleration sensor device packaging the acceleration sensor 10 thereinhas such an advantage that it is more easily handled, for example, moreeasily mounted on a specimen, compared with a unit acceleration sensor.

For the acceleration sensor, whether it is certainly fastened to thespecimen is one of the important elements to improve the detectionsensitivity. When an acceleration sensor using a deflection of apiezoelectric single crystal explained as a prior art example is storedin a package, even if the end of the vibrator is firmly fastened,characteristic deterioration does not occur. However, in the case of theacceleration sensor 10 of the present invention, since the slidingvibration of the vibrator 1 resulting from the angular moment of theweight portion 2 is used instead of deflection, if the vibrator isstrongly fastened or the fastened position or fastened length is setinappropriately, there is a possibility that the angular moment islimited and characteristic deterioration occurs.

Therefore, an acceleration sensor device of the present inventionexplained below is arranged so as to efficiently exert sliding vibrationof the vibrator 1 due to the angular moment of the weight section 2 andprevent characteristic deterioration even when the acceleration sensor10 is stored in a package. Moreover, the easiness of the packagingprocess is one of the important elements for such an acceleration sensordevice. In the acceleration sensor device of the present inventionexplained below, the acceleration sensor is simply packaged.

In the acceleration sensor device of the present invention, wiringpatterns are formed on the weight section 2 connected to the vibrator 1,and on a package including a base section and a cap section. The weightsection 2, or the weight section 2 and vibrator 1 is sandwiched by thepackage. Furthermore, the sandwiched length of the weight section 2 orvibrator 1 by the package is optimized. Therefore, it is possible toprevent characteristic deterioration of the stored acceleration sensorand to facilitate the packaging process.

(Twenty First Embodiment)

FIG. 44 is a cross sectional view showing the structure of anacceleration sensor device according to the twenty first embodiment ofthe present invention. For instance, the acceleration sensor 10explained in the first embodiment is stored in a ceramic package 60. Thepackage 60 includes a ceramic base section 61 having a cross sectionsubstantially in the shape of a square bracket and a ceramic cap section62 in the shape of a flat plate provided to cover the base section 61.The base section 61 and cap section 62 are provided with wiring patterns61 a and 62 a, respectively, and the wiring patterns 2 a, 2 a of theweight section 2 and the wiring pattern 62 a of the cap section 62 areconnected, for example, with wires 63. The output voltages from theelectrodes 1 a, 1 a are drawn to an external differential amplifier (notshown) through the wiring patterns 2 a, 2 a of the weight section 2, thewiring pattern 62 a of the cap section 62 and the wiring pattern 61 a ofthe base section 61.

(Twenty Second Embodiment)

FIG. 45 is a cross sectional view showing the structure of anacceleration sensor device according to the twenty second embodiment ofthe present invention. For instance, the single-output type accelerationsensor 10 explained in the tenth embodiment is stored in the samepackage 60 as in the twenty first embodiment. Further, a preamplifier 52is stored in the package 60 in such a manner that the preamplifier 52 isinstalled on the base section 61. The wiring pattern 2 a of the weightsection 2 and the wiring pattern 62 a of the cap section 62 areconnected, for example, by a wire 63. The output voltage from theelectrode 1 a on the free-end side is input to the preamplifier 52through the wiring pattern 2 a of the weight section 2, the wiringpattern 62 a of the cap section 62 and the wiring pattern 61 a of thebase section 61, and the resulting amplified signal is input to anexternal signal processing circuit (not shown).

(Twenty Third Embodiment)

FIGS. 46A and 46B are the cross sectional view and bottom view of theceramic base section 61, with a cross section substantially in the shapeof a square bracket, of the package 60 of an acceleration sensor deviceaccording to the twenty third embodiment of the present invention, andFIGS. 47A and 47B are the cross sectional view and bottom view of theceramic cap section 62 in the shape of a flat plate of the same package60. In the cross sectional view of FIG. 46A, a signal detection section70 provided in the base section 61 is illustrated.

Electrodes 71, 71 of the signal detection section 70 are connected viathrough-holes 72, 72 to electrodes 73, 73 for drawing signals out of thepackage 60, formed on the lower face of the base section 61. Moreover, awiring pattern 61 a formed on the inner face of the base section 61 andan electrode 74 for connection to the ground, formed on the lower faceof the base section 61, are connected via a through-hole 75. The cap 62is provided with a wiring pattern 62 a which forms a conducting pathover the entire top face, and the wiring pattern 62 a is bent to cover acertain region on the rear face of the wiring pattern 62 a so as toincrease the shielding effect. In the example shown in FIGS. 46A and46B, while the entire inner face of the base section 61 is shielded, theentire outer face thereof may be shielded. Moreover, the base section 61may be formed in a layered structure, and a signal line and ground linemay be arranged between the respective layers.

FIG. 48 is a cross sectional view showing the structure of anacceleration sensor device according to the twenty third embodiment ofthe present invention. For instance, an acceleration sensor 10 asexplained in the second example of the eleventh embodiment is stored inthe package 60 composed of the base section 61 and cap section 62 shownin FIGS. 46A, 46B and FIGS. 47A and 47B, respectively. The wiringpatterns 2 a, 2 a of the weight section 2 and the electrodes 71, 71 ofthe signal detection section 70 are bonded together by connectingmembers 76, 76 such as thermo-compression ribbon bonding, ultrasonicbonding and FPC. An electrode 1 c for connection to the ground, formedon the bottom face of the vibrator 1, and the wiring pattern 61 a of thebase section 61 are bonded together with a conductive adhesive, cream ofsolder, bump, etc. Further, the wiring pattern 61 a of the base section61 and the wiring pattern 62 a of the cap section 62 are connected.

According to the twenty third embodiment as described above, it ispossible to package an acceleration sensor by a simple process, readilyassemble an acceleration sensor device, and reduce the cost.

Further, it is possible to construct the acceleration sensor device sothat the orientation of the vibrator 1 is opposite to that shown in FIG.48, i.e., the electrode 74 for connection to the ground and the weightsection 2 are connected and the signal detection section 70 and thepackage 60 are connected. In this case, however, it is necessary to jointhe vibrator 1 and package 60 so as to exert sliding vibration of thevibrator 1.

(Twenty Fourth Embodiment: First Example and Second Example)

FIG. 49 is a cross sectional view showing the structure of anacceleration sensor device according to the first example of the twentyfourth embodiment of the present invention. For instance, stored in thepackage 60 is an acceleration sensor 10 having the characteristics ofthe second embodiment, the sixth or eleventh embodiment, the tenthembodiment and the twelfth embodiment all together, i.e., anacceleration sensor 10 in which the vibrator 1 is divided by theformation of the groove 1 b, the weight section 2 has a three-layerstructure consisting of the first weight 21 and the second weights 22,22, detection is made based on a single output from only one of theelectrodes 1 a, and the wiring patterns 2 a, 2 a on the front and rearfaces are connected with the through-hole 53.

Moreover, the first weight 21 is further extended from the top of thevibrator 1, and the extended end portion 21 d is sandwiched by thepackage 60. Besides, the wiring pattern 2 a formed on the upper face ofthe extended end portion 21 d and the wiring pattern 61 a for signaldetection of the base section 61 of the package 60 are bonded togetherwith a conductive adhesive. The package 60 has a structure where thewiring pattern 61 a for shielding is formed on the inner front face ofthe package 60.

FIG. 50 is a cross sectional view showing the structure of anacceleration sensor device according to the second example of the twentyfourth embodiment of the present invention. In the second example, ametal is used as the material of the cap section 62. Otherconfigurations are the same as those of the first example shown in FIG.49.

Since an applied acceleration acts on the center of gravity of theweight section 2, in the twenty fourth embodiment, a part of the weightsection 2, located on one side of the position of the vibrator 1opposite to the other side including the position of the center ofgravity, is sandwiched by the package 60 so as to prevent deteriorationof the detection characteristics. The weight section 2 rotates about thesupport point on the vibrator 1. Therefore, like the twenty fourthembodiment, even when a portion of the weight section 2, located on oneside of the position of the vibrator 1 opposite to the other sideincluding the position of the center of gravity, is sandwiched by thepackage 60, the angular moment of the weight section 2 is not limitedand deterioration of the detection characteristics does not occur.

FIGS. 51A and 51B are the plan view and cross sectional view of theweight section 2 of the acceleration sensor device according to thetwenty fourth embodiment. In these figures, W₂ and T₂ represent theentire width and entire thickness of the weight section 2, respectively.Besides, W₂₁ and T₂₁ represent the width and thickness of the sandwichedextended end portion 21 d, respectively. The present inventors foundthat, in the twenty fourth embodiment in which a part of the weightsection 2 is sandwiched by the package 60, the ratio of the size (widthW₂₁, thickness T₂₁) of the sandwiched extended end portion 21 d to theentire size (width W₂, thickness T₂) of the weight section 2 greatlyaffects the detection characteristics.

FIG. 52 is a graph showing the relationship between the ratio W₂₁/W₂ ofthe width W₂, of the extended end portion 21 d to the entire width W₂ ofthe weight section 2 (the horizontal axis) and the standardized outputvoltage (the vertical axis). It will be understood from the result shownin FIG. 52 that as the ratio W₂₁/W₂ increases, i.e., as the width W₂₁ ofthe sandwiched extended end portion 21 d becomes wider, the outputvoltage is lowered. In particular, when the ratio W₂₁/W₂ is not lessthan 1, since the lowering of the output voltage is noticeable, it ispreferable to make the width W₂₁ of the extended end portion 21 dnarrower than the entire width W₂ of the weight section 2 so as toprevent the deterioration of the detection characteristics.

Additionally, it was verified from the result of a simulation that thethickness T₂₁ of the extended end portion 21 d and the entire thicknessT₂ of the weight section 2 have a relationship similar to theabove-described relationship about the width, and thus it is preferableto make the thickness T₂₁ of the extended end portion 21 d thinner thanthe entire thickness T₂ of the weight section 2 so as to prevent thedeterioration of the detection characteristics.

According to the twenty fourth embodiment, by making the width of theportion sandwiched by the package 60 narrower than the entire width ofthe weight section 2 and/or making the thickness of the sandwichedportion thinner than the entire thickness of the weight section 2, it ispossible to achieve a structure which does not limit the angular momentof the weight section 2, thereby maintaining high detectioncharacteristics.

(Twenty Fifth Embodiment)

FIG. 53 is a cross sectional view showing the structure of anacceleration sensor device according to the twenty fifth embodiment ofthe present invention. For instance, like the twenty fourth embodiment,stored in the package 60 is the acceleration sensor 10 having thecharacteristics of the second embodiment, the sixth or eleventhembodiment, the tenth embodiment and the twelfth embodiment alltogether. However, unlike the twenty fourth embodiment, the supportedend of the first weight 21 and the supported end of the vibrator 1 areflush with each other, and the supported-end portion of the first weight21 and the supported-end portion of the vibrator 1 are sandwiched by thepackage 60. The characteristic of the twenty fifth embodiment is that itcan achieve a smaller size in comparison with the twenty fourthembodiment. In FIG. 53, L₃ represents the length of the sandwichedportion, and L₁ is the entire length of the vibrator 1.

The present inventors found that, in the twenty fifth embodiment inwhich the vibrator 1 and weight section 2 are partly sandwiched by thepackage 60, the ratio of the length L₃ of the sandwiched portion to theentire length L₁ of the vibrator 1 greatly affects the detectioncharacteristics. FIG. 54 is a graph showing the relationship between theratio L₃/L₁ of the length L₃ of the sandwiched portion to the entirelength L₁ of the vibrator 1 (the horizontal axis) and the standardizedoutput voltage (the vertical axis).

It will be understood from the result shown in FIG. 54 that as the ratioL₃/L₁ increases, i.e., as the length L₃ of the sandwiched portionbecomes longer, the output voltage is lowered. In particular, when theratio L₃/L₁ is not less than 1, since the output voltage is extremelylow, it is preferable to make the length L₃ of the sandwiched portionshorter than the entire length L₁ of the vibrator 1 so as to prevent thedeterioration of the detection characteristics.

By the way, in the twenty fifth embodiment, as shown in FIG. 53, one ofthe divided electrodes 1 a, which is located on the supported-end sideand does not participate in the detection of a voltage, and theelectrode 1 c for connection to the ground formed on the bottom face ofthe vibrator 1 are made the same electric potential through the wiringpattern 61 a of the base section 61. If the output voltage of anacceleration sensor device in which the electrode 1 a of the detectionportion and the electrode 1 c for connection to the ground are not ofthe same electric potential is “1”, the output voltage of the twentyfifth embodiment in which the electrodes 1 a and 1 c are of the sameelectric potential is “1.01”. Hence, it will be understood that such astructure does not affect the detection characteristics at all. With astructure as illustrated in the twenty fifth embodiment, it is possibleto simplify the wiring patterns in the weight section 2 and package 60and to provide a more simple fabrication process.

(Twenty Sixth Embodiment: First Example and Second Example)

FIG. 55 is a cross sectional view showing the structure of anacceleration sensor device according to the first example of the twentysixth embodiment of the present invention. For instance, stored in apackage 60 composed of a base section 61 in the shape of a flat plateand a cap section 62 having a cross section in the shape of a squarebracket is an acceleration sensor 10 having the characteristics of thefourth example of the sixth embodiment and the tenth embodiment alltogether, i.e., an acceleration sensor 10 which has a weight section 2constructed by engaging a first weight 21 on the supported-end side witha second weight 22 on the free-end side and performs single-output typedetection using only the output from one of the electrodes 1 a.

Moreover, the first weight 21 is further extended from the top of thevibrator 1. The base section 61 has a protruding portion 61 b at aposition corresponding to the position of the extended portion. Theextended portion of the first weight 21 is placed on this protrudingportion 61 b, and a wiring pattern 2 a of the first weight 21 and awiring pattern 61 a formed on the protruding portion 61 b are connected.In this case, the wiring pattern 61 a of the protruding portion 61 bjust needs to be electrically connected, it is possible to use, forexample, a silver paste.

FIG. 56 is a cross sectional view showing the structure of anacceleration sensor device according to the second example of the twentysixth embodiment of the present invention. In the second example, ametal is used as the material of the cap section 62. Otherconfigurations are the same as those of the first example shown in FIG.55. In comparison with the first example using the ceramic cap section62, the second example can make the cap section 62 thinner, therebyenabling a reduction in the height of the device. Alternatively, if aresin cap section 62 is used, it is possible to reduce the cost.

According to the structure of the twenty sixth embodiment, for example,if the height of the protruding portion 61 b is formed to a negativetolerance with respect to the thickness of the vibrator 1, it is notnecessary to particularly specify the thickness of the vibrator 1 andweight section 2. Besides, it is necessary to simply place the vibrator1 and weight section 2 sequentially on the base section 61 in the shapeof a flat plate and to finally put the cap section 62 thereon, and thushandling of the vibrator 1 and weight section 2 is easier and thefabrication process is simplified.

Incidentally, while the above description explains a structure in whichthe base section 61 in the shape of a flat plate is provided with theprotruding portion 61 b, needless to say, the same effect is alsoobtained by a structure including a protruding portion or a structureequivalent to the protruding portion in the weight section 2.

(Twenty Seventh Embodiment)

FIGS. 57A and 57B are plan views of the package 60 side of the vibrator1 and the vibrator 1 side of the base section 61 of the package 60 of anacceleration sensor device according to the twenty seventh embodiment.The twenty seventh embodiment is configured by applying the bondingrelationship between the vibrator 1 and the weight section 2 of theabove-mentioned nineteenth embodiment to the bonding relationshipbetween the vibrator 1 and the package 60 (the base section 61). An areaenclosed by the broken line in FIG. 57B is a region where the vibrator 1is to be bonded. The area of the electrode 1 c of the vibrator 1 and thewiring pattern 61 a of the base section 61 is smaller than the bondedarea between the vibrator 1 and the package 60 (the base section 61).

When gold is used for the electrode 1 c and wiring pattern 61 a, thereis a possibility that the detection performance deteriorates for thesame reasons as in the nineteenth embodiment. Therefore, in the twentyseventh embodiment, the area of the electrode 1 c and wiring pattern 61a is made smaller than the bonded area so as to obtain a sufficientbonding strength in other region. Accordingly, even when the electrode 1c and wiring pattern 61 a do not easily stick to the adhesive, it ispossible to increase the bonding strength between the vibrator 1 and thepackage 60 (the base section 61).

Further, in the case where the area of the electrode 1 c and wiringpattern 61 a is smaller than the bonded area, the electrode 1 c andwiring pattern 61 a may have an arbitrary shape. FIGS. 58A and 58B areplan views of the vibrator 1 side of the base section 61 of theacceleration sensor device according to the twenty seventh embodiment.Besides, in order to increase the degree of adhesion between thevibrator 1 and the package 60 (the base section 61), it is preferable toarrange the electrode 1 c and wiring pattern 61 a so that they aresymmetrical about a line, symmetrical about a point and supported atthree points.

(Twenty Eighth Embodiment)

FIG. 59 is a cross sectional view showing the structure of anacceleration sensor device according to the twenty eighth embodiment ofthe present invention. Stored in a package 60 having substantially thesame structure as that of the above-described twenty sixth embodiment isan acceleration sensor 10 having the characteristic of theabove-described twentieth embodiment, i.e., an acceleration sensor 10 inwhich the supported-end side of the weight section 2 has a cavitystructure.

The wiring pattern 2 a of the first weight 21 and the wiring pattern 61a of the base section 61 are connected with a conductive paste 91 madeof, for example, a silver paste. Thus, even though the accelerationsensor 10 has the cavity 90, the readiness of drawing a detection signalto an external device is not impaired.

Incidentally, while this example is constructed such that the detectionsignal is drawn to an external device through the conductive paste 91,the wiring patterns 2 a and 61 a may be connected by wire bonding asanother method satisfying the conditions for not limiting slidingvibration during the application of an acceleration and the detectionsignal may be drawn to an external device through the wire bonding.

Further, in each of the acceleration sensor devices explained above, thestructure of the acceleration sensor stored in the package 60 is only anexample and, needless to say, an acceleration sensor having otherstructure explained by the present invention can be stored in the samemanner.

By the way, if a positioning structure for mounting the vibrator 1 onthe weight section 2, package 60 or specimen 4 is provided, thepositioning of the mutual members can be easily and accuratelyperformed, thereby facilitating the fabrication process and reducingvariations in the detection characteristics.

As this invention may be embodied in several forms without departingfrom the spirit of essential characteristics thereof, the presentembodiments are therefore illustrative and not restrictive, since thescope of the invention is defined by the appended claims rather than bythe description preceding them, and all changes that fall within metesand bounds of the claims, or equivalence of such metes and boundsthereof are therefore intended to be embraced by the claims.

What is claimed is:
 1. An acceleration sensor device comprising: anacceleration sensor including a piezoelectric element provided with arespective electrode and subject to a shearing strains, and a weightsection connected to said piezoelectric element and supported at asupport point different from a position of a center of gravity of anassembly of said piezoelectric element and weight section, wherein saidweight section is provided with a wiring pattern connected to each saidelectrode, wherein said piezoelectric element detects an angular momentabout the support point as shearing strains, the angular moment beingexerted in said weight section when an acceleration is applied, andelectrical signals corresponding to said acceleration are output fromeach said electrode through said wiring pattern; and a package includinga base provided with said wiring pattern and a cap covering said base,for storing said acceleration sensor therein.
 2. The acceleration sensordevice as set forth in claim 1, wherein said package has a detectioncircuit for detecting an acceleration based on said electrical signal.3. The acceleration sensor device as set forth in claim 1, wherein apart of said weight section is sandwiched by said package.
 4. Theacceleration sensor device as set forth in claim 1, wherein a part ofsaid weight section and a part of said piezoelectric element aresandwiched by said package.
 5. The acceleration sensor device as setforth in claim 3, wherein the part of said weight section sandwiched bysaid package is a portion located on one side of the position of saidpiezoelectric element opposite to the other side including the positionof said center of gravity.
 6. The acceleration sensor device as setforth in claim 3, wherein a length of the part of said weight sectionsandwiched by said package is not more than a length of saidpiezoelectric element.
 7. The acceleration sensor device as set forth inclaim 1, wherein said piezoelectric element has a package-sideelectrode, said package is provided with a wiring pattern connected tosaid package-side electrode, said piezoelectric element and package arebonded together with an adhesive, and an area of their bonded face islarger than an area of said package-side electrode and/or said wiringpattern.